It has been known in the past to arrange a catalyst able to store oxygen in an exhaust passage of an internal combustion engine and to remove unburned gas (HC, CO, etc.) and NOX in the exhaust gas at the catalyst. The higher the oxygen storage ability of the catalyst, the greater the amount of oxygen able to be stored at the catalyst and the more the exhaust purification performance of the catalyst is improved.
To maintain the oxygen storage ability of the catalyst, it is preferable to make the oxygen storage amount of the catalyst fluctuate so that the oxygen storage amount of the catalyst is not maintained constant. In the internal combustion engine described in PTL 1, an upstream side catalyst and a downstream side catalyst are arranged in the exhaust passage, and the oxygen storage amount of the upstream side catalyst is made to fluctuate by two types of control: one-sided failure control and two-sided failure control. In the one-sided failure control and two-sided failure control, the target air-fuel ratio of the exhaust gas flowing into the upstream side catalyst is alternately switched between a lean air-fuel ratio leaner than a stoichiometric air-fuel ratio and a rich air-fuel ratio richer than a stoichiometric air-fuel ratio based on an output of a downstream side air-fuel ratio sensor arranged at a downstream side of the upstream side catalyst etc.
Specifically, in two-sided failure control, when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio or becomes less, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio, while when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a lean judged air-fuel ratio leaner than the stoichiometric air-fuel ratio or becomes more, the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio. Therefore, in two-sided failure control, the oxygen storage amount of the upstream side catalyst fluctuates between zero and the maximum oxygen storage amount. For this reason, the oxygen storage amount of the upstream side catalyst periodically becomes zero or the maximum oxygen storage amount and small amounts of unburned gas and NOX are exhausted from the upstream side catalyst.
On the other hand, in one-sided failure control, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio richer than the stoichiometric air-fuel ratio or becomes less, while the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio when the oxygen storage amount of the upstream side catalyst becomes the switching storage amount or becomes more. Therefore, in one-sided control, the oxygen storage amount of the upstream side catalyst fluctuates between zero and the switching storage amount. For this reason, the oxygen storage amount of the upstream side catalyst periodically becomes zero and a small amount of unburned gas is exhausted from the upstream side catalyst.
Further, in one-sided failure control, the oxygen storage amount of the upstream side catalyst is calculated based on the output of the upstream side air-fuel ratio sensor arranged at the upstream side of the upstream side catalyst. For this reason, even if the maximum oxygen storage amount of the upstream side catalyst becomes less than the switching storage amount due to deterioration etc., the target air-fuel ratio is maintained at the lean air-fuel ratio until the calculated oxygen storage amount reaches the switching storage amount. As a result, after the oxygen storage amount of the upstream side catalyst reaches the maximum oxygen storage amount, a large amount of NOX flows out from the upstream side catalyst and the exhaust emission deteriorates.
In the internal combustion engine described in PTL 1, to solve this technical problem, one-sided failure control is performed when the maximum oxygen storage amount of the upstream side catalyst is equal to or larger than a predetermined value, and two-sided failure control is performed when the maximum oxygen storage amount of the upstream side catalyst is less than a predetermined value.